1. Field of the Invention
The present invention relates to a protecting apparatus for an inverter constituted by a plurality of switching devices connected between an anode and a cathode of a DC power source.
2. Description of the Related Art
Hitherto, a motor for driving a hermetic electric compressor constituting, for example, an air conditioner or a refrigerating cycle of an electric refrigerator or the like, a synchronous motor that has a permanent magnet in its rotor and is driven by direct current, or an induction motor driven by alternating current has been adopted. In recent years, a synchronous induction motor having a permanent magnet for its rotor and squirrel-cage secondary conductors have also been used. These motors are driven by inverters.
A drive circuit of such a conventional motor will now be described with reference to FIG. 10. Shown in the drawing are an inverter main circuit power source Vcc1, an inverter 101 that generates three-phase pulse width modulation (PWM) outputs by a plurality of high-frequency output switching devices Tr1, Tr2, Tr3, Tr4, Tr5, and Tr6, a motor 102 installed in an electric compressor (not shown), a drive circuit 103, a microcomputer 104, an operational amplifier AMP, and current transformers 105 and 106 for detecting currents passing through the motor 102. The microcomputer 104 generates drive signals to cause the motor 102 to continuously rotate, and drives the inverter 101 through the intermediary of the drive circuit 103. The inverter 101 generates three-phase PWM substantially sinusoidal waveforms, using a series connection point, as an output, of each pair of the switching devices Tr1 and Tr4, Tr2 and Tr5, and Tr3 and Tr6 that are connected in parallel to the power source Vcc1, thereby continuously running the motor 102 at a predetermined number of revolutions.
The currents of two phases, namely, the current output between the switching devices Tr1 and Tr4 and the current output between the switching devices Tr2 and Tr5 are detected by the current transformers 105 and 106. The detection outputs are amplified by the operational amplifiers AMP and AMP, then supplied to the microcomputer 104. The microcomputer 104 presumes the position of the rotor of the motor 102 on the basis of the detection outputs of the current transformers 105 and 106, controls the drive circuit 103 to drive the inverter 101, and generates the three-phase PWM substantially sinusoidal waveforms to rotatively control the motor 102.
If the detection outputs of the current transformers 105 and 106 are higher than a preset value, i.e., if overcurrent flows from the inverter 101 to the motor 102, the microcomputer 104 controls the drive circuit 103 to stop the inverter 101 and also stop the motor 102 so as to protect the inverter 101 and the motor 102 from overcurrent.
Thus, in the prior art, the output currents of the inverter 101 are detected by the current transformers 105 and 106 to presume the position of the rotor of the motor 102 and to protect the motor 102 from overcurrent. The current transformers, however, are expensive; therefore, as shown in FIG. 10, current transformers for only two phases are usually inserted, and the microcomputer 104 has to compute a formula IT=xe2x88x92(IR+IS) to calculate the current value for the remaining one phase (e.g., T phase).
The present invention has been made with a view toward solving the problem with the prior art, and it is an object of the invention to provide a protecting apparatus capable of securely and inexpensively effecting protection of an inverter from overcurrent.
According to one aspect of the present invention, there is provided a protecting apparatus that is applied to an inverter constituted by a plurality of switching devices connected between an anode and a cathode of a DC power source, and includes a plurality of detecting devices that convert the currents passing through the plurality of switching devices into voltages and generate detection outputs, a plurality of comparing devices that receive the detection outputs of the detection devices and generate an abnormality output if the absolute values of the detection outputs exceed a predetermined value, and a protecting device that interrupts the drive of all the switching devices if any of the comparing devices generates the abnormality output.
In a preferred form of the present invention, the inverter is constructed of a plurality of pairs of the switching devices connected in parallel between the anode and the cathode of the DC power source, each of the pairs being formed of the switching devices connected in series. The inverter uses, as outputs, the connection points of the switching devices of the pairs mentioned above to convert DC to AC. The detecting devices convert the currents passing through the pairs of switching devices connected to the cathode into voltages, and generate detection outputs.
In another preferred form, the protecting device interrupts the drive of the switching devices if an abnormal output from the comparing devices occurs and lasts for a predetermined period of time.
In yet another preferred form, if the drive of the switching devices is interrupted, the protecting device maintains the interruption state until a predetermined release instruction is issued.
In still another preferred form, the detection outputs of the detecting devices are the information for detecting the condition of a motor driven by the inverter.
In a further preferred form, the motor is a synchronous motor having a permanent magnet in its rotor.
In another preferred form, the motor is a synchronous induction motor having a permanent magnet in its rotor.
In another preferred form, the motor is an induction motor.